To prevent removal of printed images from porous or semiporous substrates, it is desired that the images are indelible. The images printed on porous or semiporous substrates need to resist attempts to remove or wash out the image (mark) by using aqueous or nonaqueous liquids.
An ink that is currently used to print messages contains a “solvent dye,” in the vernacular of the Colour Index that is insoluble in water, but soluble in organic solvents. The image printed from this ink on porous or semiporous substrates resists removal by washing in aqueous liquids, but is not resistant to removal if washed with nonaqueous liquids. Indelibility to nonaqueous liquids is needed.
Ink jet printing is well known. Typically, there are two types of ink jet printing systems, the continuous type and the drop-on-demand type. In the continuous ink jet printing system, the droplets of the ink are passed through a charging area wherein individual droplets receive an electrical charge in response to a signal. The droplets are then passed through an electric field, causing a varied deflection of the individual droplets dependent on the intensity of the charge and the field. The droplets are produced continuously, and are directed continuously to the substrate to be printed on or, alternatively, to a bypass gutter. In the drop-on-demand system, the ink droplets are expelled from a printing head only when required during the printing process. Such inks need not be electrically conductive.
Therefore, there exists a need for an ink composition, particularly for use in a drop-on-demand printer, suitable for printing indelible images on porous, particularly semiporous substrates.
The invention provides such an ink composition. The advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.